Prefix discovery –
Hosts discover the set of address prefixes that define which destinations
are attached to the link (sometimes referred to as on-link). Nodes use prefixes
to distinguish destinations that reside on a link from those only reachable
through a router.

Parameter discovery –
A node learns link parameters, such as the link maximum transmission unit
(MTU). A node also learns Internet parameters, such as the hop limit value,
to place in outgoing packets.

Address resolution –
Nodes determine the link-layer address of a neighbor (an on-link destination)
with only the destinations's IP address.

Next-hop determination –
An algorithm determines mapping for an IP destination address into the IP
address of the neighbor to which traffic for the destination should be sent.
The next-hop can be a router or the destination.

Neighbor unreachability detection – Nodes determine that a neighbor is no longer reachable.
For neighbors that are used as routers, alternate default routers can be
tried. For both routers and hosts, address resolution can be performed again.

Duplicate address detection –
A node determines that an address that the node wants to use is not already
in use by another node.

Redirect – A router
informs a host of a better first-hop node to reach a particular destination.

Neighbor discovery defines five different Internet Control Message Protocol
(ICMP) packet types: a pair of router solicitation and router advertisement
messages, a pair of neighbor solicitation and neighbor advertisements messages,
and a redirect message. The messages serve the following purpose:

Router solicitation –
When an interface becomes enabled, hosts can send router solicitations. The
solicitations request routers to generate router advertisements immediately,
rather than at their next scheduled time.

Router advertisement –
Routers advertise their presence, various link parameters, and various Internet
parameters. Routers advertise either periodically, or in response to a router
solicitation message. Router advertisements contain prefixes that are used
for on-link determination or address configuration, a suggested hop limit
value, and so on.

Neighbor solicitation –
Sent by a node to determine the link-layer address of a neighbor. Also, sent
by a node to verify that a neighbor is still reachable by a cached link-layer
address. Neighbor solicitations are also used for duplicate address detection.

Neighbor advertisement –
A response to a Neighbor Solicitation message, node can also send unsolicited
neighbor advertisements to announce a link-layer address change.

Redirect – Used by
routers to inform hosts of a better first hop for a destination, or that the
destination is on-link.

Router Advertisement

On multicast-capable links and point-to-point links, each router periodically
multicasts a router advertisement packet that announces its availability.
A host receives router advertisements from all routers, building a list of
default routers. Routers generate router advertisements frequently enough
that hosts learn of their presence within a few minutes. However, routers
do not advertise frequently enough to rely on an absence of advertisements
to detect router failure. A separate detection algorithm that determines neighbor
unreachability provides failure detection.

Router Advertisement Prefixes

Router advertisements contain a list of prefixes that is used for on-link
determination. The list of prefixes is also used for autonomous address configuration.
Flags that are associated with the prefixes specify the intended uses of a
particular prefix. Hosts use the advertised on-link prefixes to build and
maintain a list. The list is used to decide when a packet's destination is
on-link or beyond a router. A destination can be on-link even though the destination
is not covered by any advertised on-link prefix. In such instances, a router
can send a redirect that informs the sender that the destination is a neighbor.

Router Advertisement Messages

Router advertisement messages also contain Internet parameters, such
as the hop limit that hosts should use in outgoing packets. Optionally, router
advertisement messages also contain link parameters, such as the link MTU.
This feature enables centralized administration of critical parameters. The
parameters can be set on routers and automatically propagated to all hosts
that are attached.

Nodes accomplish address
resolution by multicasting a neighbor solicitation that asks the target node
to return its link-layer address. Neighbor solicitation messages are multicast
to the solicited-node multicast address of the target address. The target
returns its link-layer address in a unicast neighbor advertisement message.
A single request-response pair of packets is sufficient for both the initiator
and the target to resolve each other's link-layer addresses. The initiator
includes its link-layer address in the neighbor solicitation.

Neighbor Solicitation and Unreachability

Neighbor solicitation messages can also be used to determine if more
than one node has been assigned the same unicast address.

Neighbor unreachability detection detects the
failure of a neighbor or the failure of the forward path to the neighbor.
This detection requires positive confirmation that packets that are sent to
a neighbor are actually reaching that neighbor and being processed properly
by its IP layer. Neighbor unreachability detection uses confirmation from
two sources. When possible, upper-layer protocols provide a positive confirmation
that a connection is making forward progress. That is,
data that was sent previously is known to have been delivered correctly. For
example, new TCP acknowledgments were received recently. When positive confirmation
is not forthcoming through such hints, a node sends unicast neighbor solicitation
messages. These messages solicit neighbor advertisements as reachability confirmation
from the next hop. To reduce unnecessary network traffic, probe messages are
sent only to neighbors to which the node is actively sending packets.

In addition to addressing the previous general problems, neighbor discovery
also handles the following situations.

Link-layer address change –
A node that knows its link-layer address has been changed can multicast a
few (unsolicited) neighbor advertisement packets. The node can multicast to
all nodes to update cached link-layer addresses that have become invalid.
The sending of unsolicited advertisements is a performance enhancement only.
The detection algorithm for neighbor unreachability ensures that all nodes
reliably discover the new address, though the delay might be somewhat longer.

Inbound load balancing –
Nodes with replicated interfaces might want to load-balance the reception
of incoming packets across multiple network interfaces on the same link. Such
nodes have multiple link-layer addresses assigned to the same interface. For
example, a single network driver can represent multiple network interface
cards as a single logical interface that has multiple link-layer addresses.

Load balancing is handled by allowing routers to omit the source link-layer
address from router advertisement packets. Consequently, neighbors must use
neighbor solicitation messages to learn link-layer addresses of routers. Returned
neighbor advertisement messages can then contain link-layer addresses that
differ, depending on who issued the solicitation.

Anycast addresses –
Anycast addresses identify one of a set of nodes that provide an equivalent
service. Multiple nodes on the same link can be configured to recognize the
same anycast address. Neighbor discovery handles anycasts by setting nodes
to expect to receive multiple neighbor advertisements for the same target.
All advertisements for anycast addresses are tagged as being non-override
advertisements. Non-override advertisements invoke specific rules to determine
which of potentially multiple advertisements should be used.

Proxy advertisements –
A router that accept packets on behalf of a target address that is unable
to respond to neighbor solicitations can issue non-override neighbor advertisements.
Currently, the use of proxy is not specified. However, proxy advertising can
potentially be used to handle cases like mobile nodes that have moved off-link.
However, the use of proxy is not intended as a general mechanism to handle
nodes that do not implement this protocol.

Comparison With IPv4

The IPv6 neighbor discovery protocol corresponds to a combination of
the IPv4 protocols Address Resolution Protocol (ARP), ICMP Router Discovery,
and ICMP Redirect. IPv4 does not have a generally agreed on protocol or mechanism
for neighbor unreachability detection. However, host requirements do specify
some possible algorithms for dead gateway detection. Dead gateway detection
is a subset of the problems that neighbor unreachability detection solves.

The neighbor discovery protocol provides a multitude of
improvements over the IPv4 set of protocols.

Router discovery is part of the base protocol set. Hosts do
not need to snoop the routing protocols.

Router advertisements carry prefixes for a link. A separate
mechanism is not needed to configure the netmask.

Router advertisements enable address autoconfiguration.

Routers can advertise an MTU for hosts to use on the link.
Consequently, all nodes use the same MTU value on links that lack a well-defined
MTU.

Address resolution multicasts are spread over 4 billion (2^32)
multicast addresses, greatly reducing address-resolution-related interrupts
on nodes other than the target. Moreover, non-IPv6 machines should not be
interrupted at all.

Redirects contain the link-layer address of the new first
hop. Separate address resolution is not needed on receiving a redirect.

Multiple prefixes can be associated with the same link. By
default, hosts learn all on-link prefixes from router advertisements. However,
routers can be configured to omit some or all prefixes from router advertisements.
In such instances, hosts assume that destinations are off-link. Consequently,
hosts send the traffic to routers. A router can then issue redirects as appropriate.

Unlike IPv4, the recipient of an IPv6 redirect assumes that
the new next-hop is on-link. In IPv4, a host ignores redirects that specify
a next-hop that is not on-link,
according to the link's network mask. The IPv6 redirect mechanism is analogous to the XRedirect facility.
The redirect mechanism is useful on non-broadcast and shared media links.
On these links, it is undesirable or not possible for nodes to check for all
prefixes for on-link destinations.

Neighbor unreachability detection improves packet delivery
in the presence of failing routers. This capability improves packet delivery
over partially failing or partitioned links. This capability also improves
packet delivery over nodes that change their link-layer addresses. For instance,
mobile nodes can move off-link without losing any connectivity because of
stale ARP caches.

Unlike in IPv4 router discovery, the router advertisement
messages do not contain a preference field. The preference field is not needed
to handle routers of different stability. The neighbor unreachability detection
detects dead routers and switches to a working router.

By using link-local addresses to uniquely identify routers,
hosts can maintain the router associations. The ability to identify routers
is required for router advertisements and is required for redirect messages.
Hosts need to maintain router associations if the site uses new global prefixes.

Because neighbor discovery messages have a hop limit of 255
upon receipt, the protocol is immune to spoofing attacks originating from
off-link nodes. In contrast, IPv4 off-link nodes can send Internet Control
Message Protocol (ICMP) redirects and can send router advertisement messages.

By placing address resolution at the ICMP layer, the protocol
becomes more media independent than ARP. Consequently, standard IP authentication
and security mechanisms can be used.